The oil and gas industry has always treated the collection of formation gas and formation cuttings data as one function. On an oil and gas drilling project, there are two different applications. The first drilling application is the collection of formation gases and cuttings while the drilling fluid is directed to the reserve pits. The second drilling application is the collection of formation gases and cuttings while the drilling mud is directed to the steel pits.
The traditional configuration begins at the well head. From the well head, there is a flow line in which the drilling fluid leaves the well bore. The flow line is commonly 160 mm or 225 mm (six or eight inches) in diameter. The flow line extends from the well head out towards the steel and reserve pits, where there are control valves that control the direction of the drilling fluid, either to the reserve pits, or to the steel pits. The reserve pits are located further away from the well bore and behind the steel pits. The drilling fluid from the well head carries the formation gases and formation cuttings from the well bore, to either the reserve pits, or the steel pits. The formation cuttings are dumped into the reserve pits, where they cannot reenter the drilling system. The formation gases are either released into the atmosphere, or flared off. The drilling fluid is then circulated around into the well bore, where the process is restarted.
Drilling Phase One—Water/Reserve Pits—At the beginning of a drilling project, the 30 drilling fluid is either fresh water or brine water. The drilling fluid is bypassed the steel pits and out to the reserve pits, which are located behind the steel pits. The reserve pits are earth dug pits, in the shape of a horseshoe and at a slight angle. This allows the formation cuttings to be dropped out of the fluid. The fluid flows around to the other end of the horseshoe where it is suctioned back into the well bore. The flow line is an open ended system, so there is no any pressure on the flow line. The flow line is filled approximately halfway with drilling halfway with drilling fluid. The drilling fluid is a mixture of formation gases and formation cuttings. The top half of the flow line is filled with formation gases.
Current practice in the industry is to insert a two-inch line into the flow of drilling mud, diverting it to the formation cuttings sample box that is installed near the end of the drilling fluid flow line and installed low enough, relative to the drilling mud source, so the fluid flows easily into the formation sample box.
The formation cutting sample box is a rectangular box, with a sliding door in the front.
The formation cutting sample box is designed for catching formation cuttings, as well as holding enough drilling fluid for the agitator, for monitoring formation gas. Its width and length must be wide enough, to allow a sample box agitator stand to fit inside, and long enough, as not to hinder the collection of formation cuttings to be analyzed. It must also be built sturdy enough to withstand the vibration from the agitating motor, as well as the combined weight of the agitating stand and motor. The sliding door has a handle cut along its top edge. The handle is to allow excess fluid to flow out of the sample box, out into the reserve pits and not over the sides or end of the sample box. The sliding door is also used for washing formation cuttings out into the reserve pits, after a sample is collected, so that the next ten foot sample can be caught inside the sample box.
The sample box agitating stand is a steel stand about three to four feet tall, onto which the agitator motor is mounted. The stand has an entrance and exit portal in it that allows drilling fluid to enter and leave the box. An explosion-proof electrical agitator motor is mounted on the box which rotates beaters affixed to the motor's rotating shaft. A suction hole is drilled in the sample box to allow formation gases to be sucked out of the sample box to be analyzed.
Drilling Phase Two—Mudding Up/Steel Pits—At some point in the drilling process, the crews will begin to “mud up”, a term used by the oil and gas industry to describe the process of adding chemicals to the drilling fluid to control the properties of the drilling mud. At this point, the drilling fluid is now referred to as drilling mud. Once the determination has been made to start mudding up, the two valves are turned in the drilling fluid flow line and the flow is diverted from the reserve pits to the steel pits. Then chemicals are mixed to start the mudding up process. The drilling mud is directed to the steel pits to: 1) begin the mudding up process, 2) prevent loss of expensive drilling mud, 3) to maintain, control, and change the properties of the drilling mud, 4) to protect the well bore, and 5) to prevent or control lost circulation.
During the second phase, when drilling mud is used, the agitator stand is placed inside the sample box, at a lower bottom of a large vat, known as the possum belly, located in front of the shaker. The drilling fluid flow line enters into the possum belly at its base. The drilling mud fills the possum belly, until the drilling mud spills over the front edge, onto the shaker. The shaker includes screens and vibrates very rapidly. The drilling mud and formation cuttings spill onto the screens. The vibration of the shaker allows the drilling mud to fall through the screens, into the steel pits, leaving the formation cuttings on the screens. The drilling mud is remixed and suctioned back into the well bore. The formation cuttings are vibrated to the end of the shaker where they fall onto a slide. A sample of the formation cuttings is collected off the slide for examination. Formation gases are collected for monitoring at the possum belly. The remainder is washed off the slide, into the reserve pits.
The traditional method beats gases trapped out of the drilling fluid that is collected in the sample box, and only those gases. The sample box has to be moved when the drilling starts the mudding up process. The gas in the top half of the drilling fluid flow line simply escapes into the atmosphere and is never analyzed. Formation cuttings fill the formation cuttings sample box, plugging the hole at the base of the agitator bracket, and cause improper formation gas readings. The end result is that formation gases either cannot be monitored at all, or have very inaccurate readings. The traditional method also uses an agitating motor that vibrates, rusts, and requires electrical power in an outdoor installation. In such an environment, loss of power or a rusted motor renders the sampling system unusable.
The oil industry needs to be able to more efficiently sample the gases coming up out of the well with the drilling fluid.